r2 C. Bww — Hot, it i,>n of Int, rfi /•< in-, Fringes 



If at the iirst grating tf, = 0',, 



2a SB t — <t = 6, — 0./ + 2<£. 



The images are at an angle /3, where 



(3 = 2 (a - *) = 6 t - 6 t '. 



If G'G' is displaced to G'G' over a normal distance e or 

 e/cos<j> along the line of symmetry GT, the virtual images 



Fig. 5. 



vr 



6r m "and G n will be displaced to G' m and G' n over the same 

 normal distance e. This is obvious, since the quadrilaterals ab 

 and a'b' are rhombuses by the law of reflection, and have the 

 perpendicular distance e between the (equal) sides all identical. 

 If D. 2 is the grating space of G', 



sin 0, + sin £ = A/Z> 5 

 sin 2 '+ sin*'= A/i> 5 



0) 



or if i and £' are very nearly equal and both small, as in the 

 experiment, 



cos 6„ dO = — cos idi. 



(2) 



Again in case of a displacement e of G', the paths are short- 

 ened at G m by 



e 

 cos 0„ 



